Of the Lady's Star-glass...

[Inziladun]

Originally Posted by Formendacil

In other words--it may not have been Sauron's power at all that quenched the Phial, but the fact that it was in the heart of Orodruin. We know that Orodruin was a place of special power--that's why Sauron forged the Ring there. I wouldn't go so far as to cite the fact that it could destroy the Ring as evidence of its power, since that is tied to the fact that it was forged there, but it definitely seems to have been forged there because of a unique potency.

The narrative in ROTK telling of Sam going inside the Mountain seems to indicate it was both Sauron and the volcano itself that caused the phial to diminish in power.

In his great need [Sam] drew out once more the phial of Galadriel, but it was pale and cold in his trembling hand and threw no light into that stifling dark. He was come to the heart of the realm of Sauron and the forges of his ancient might, greatest in Middle-earth; all other powers were here subdued.

The inference is that some power, which could indeed have been something derived from Morgoth, combined with Sauron's might to keep the phial dark. The fact that the light from the Trees as contained in the Silmarils could not be darkened even in the presence of Morgoth, and the phial was dimmed in Mordor, can be explained, I think, along the lines of what Formendacil said.

Fëanor gathered the light of the Trees when it was fresh and clear, and from very close up, one would think.
Galadriel was forced to capture the light at much greater removes, from the heavens, and a very long time after the Silmaril had been set to be a star, so it doesn't seem all that odd to me that the potency of the phial would have been less. Also, great as Galadriel was, Fëanor was on an altogether different level when it came to making things like that, I would say.